Primary care offers a familiar and accessible clinical venue for patients affected by obesity to receive evidence- based lifestyle interventions for weight management. However, there are numerous barriers to the implementation of such programs in primary care, and previous interventions delivered by primary care physicians or other members of the healthcare team have often demonstrated modest and temporary weight loss for patients. Weight loss treatment delivered by trained peer coaches may offer a viable and effective alternative to address weight management in primary care. While peer coaches have successfully delivered behavioral interventions for other chronic conditions (e.g., type 2 diabetes) as well as obesity interventions in non-clinical settings (e.g., community centers), research is lacking on the effectiveness of translating peer- delivered weight loss interventions to primary care. Our preliminary work suggests that a weight loss intervention delivered in primary care by peer coaches can achieve clinically significant reductions in weight, is feasible for adoption in primary care, and is well-received by patients. In this application, we propose to conduct an 18-month randomized controlled trial comparing the effects of obesity treatment that includes either: 1) in-person and telephone-based contacts with peer coaches (peer coach treatment), or 2) routine contact with a primary care provider plus self-directed intervention materials (standard care). This weight loss trial will include 375 obese adults randomized from 10 primary care practices. The primary outcome will be changes in body weight at month 18. Secondary outcomes will include key patient-centered outcomes, including quality-of-life, physical and social functioning, mood, and treatment satisfaction. The cost- effectiveness of the peer coach intervention will also be evaluated. This is a novel investigation of the effectiveness and sustainability of an innovative alternative for obesity treatment delivery in primary care. If the peer coach intervention proves effective, it would have significant clinical and public health implications for patients, clinicians, and health systems. These potential benefits include: 1) improvement of clinical and psychosocial outcomes for patients, 2) increased treatment options for primary care providers to manage the care of obese patients, and 3) reduced healthcare utilization and costs.